A drunk driving investigation often begins with the officer noticing an indication of possible impairment such as the odor of alcohol or bloodshot eyes. However, these types of clues alone don't necessarily indicate the motorist is too impaired to lawfully drive a vehicle. So officers use a series of tasks called "field sobriety tests" (FSTs) to gauge whether a driver is safe to operate a vehicle.
This article outlines commonly used field sobriety tests and how the results can be used in DUI investigations and in court.
Generally, field sobriety tests fall into one of two categories: standardized or non-standardized. In most cases, officers use standardized tests. Research has shown the standardized field sobriety tests can be used by law enforcement to accurately assess a driver's level of impairment (more on this below). With non-standardized tests, there's generally no solid evidence indicating they're accurate.
The three standardized field sobriety tests are: the one-leg stand, the walk-and-turn, and the horizontal gaze nystagmus. Before administering any of these tests, the officer is supposed to clearly instruct the driver on how to perform the test and make sure conditions are appropriate for accurate results. As an example, prior to administering the one-leg-stand, the officer should find a flat surface, give the driver the instructions, and confirm that the driver understands all instructions before beginning. While the driver is performing the particular test, the officer grades the performance based on certain identifiable "clues" of impairment.
Here's some more specific information on the three standardized tests:
For a variety of reasons, officers sometimes use non-standardized field sobriety tests during DUI investigations. There are lots of different non-standardized tests, but they typically involve simple tasks that require dexterity or split focus. For example, an officer might ask the driver to count up to four and back down to one while saying the numbers out loud and touching their fingers to their thumb. This seemingly simple task allows the officer to see if the driver has difficulty focusing on two things at once (counting out loud and touching fingers) and with basic motor function.
Though not as conclusive as standardized tests, non-standardized tests might still help an officer determine if a driver is safe to operate a vehicle. With non-standardized tests, officers basically employ common sense while observing these tasks to determine if the driver's performance might indicate impairment.
Due to the increase of drivers impaired by drugs or other controlled substances, officers in many areas are now being trained in using field sobriety tests and their observations to recognize drug-related impairment in drivers. To assess drug impairment, officers use both standardized and non-standardized tests, but the training focuses on specific clues related to drug use. For example, certain drugs affect displacement and cause the person to veer slightly to the left when walking a straight line. Others drugs cause excessive dry mouth, so as an indication of this type of drug use, the officer might note the driver repeatedly asking for water, licking their lips, and the like.
Most drivers know police use field sobriety tests in DUI investigations. But are field sobriety tests accurate?
According to the Nation Highway Traffic Safety Administration (NHTSA), are accurate indicators of whether a driver has a blood alcohol concentration (BAC) of .1% or more. In other words, NHTSA research showed drivers with a BAC of at least .1% were likely to fail FSTs.
However, every situation is different. Failing an FST doesn't necessarily mean the driver is under the influence of alcohol. Drivers with certain medical or physical conditions might not be able to perform FSTs properly. And an officer's failure to follow proper FST procedures can also taint the results.
While conducting a DUI investigation, police are basically trying to determine whether there's enough evidence for probable cause to make an arrest. In deciding whether to make an arrest or release a driver, police look at a variety of factors, which often include FST performance. In other words, a driver's poor FST performance can lead to an arrest, whereas police might decide to release a driver who does well on FSTs.
When officers place a driver under arrest for driving under the influence, the next step is generally chemical testing (usually, a blood, breath, or urine test). State implied consent laws require all drivers lawfully arrested for a DUI to submit to chemical testing at the request of an officer.
Generally, drivers have the right to refuse to participate in field sobriety tests. In other words, FSTs are optional and drivers can't be penalized for not participating. However, under the laws of some states, refusing to perform the FSTs can be used against the driver in court. In other words, prosecutors can argue the driver's refusal shows a consciousness of guilt.
Per se DUI charges. In many cases, FST results aren't the primary evidence used in court. In every state, it's illegal to drive with a BAC of .08% or more (.05% or more in Utah). A DUI based on an excessive BAC is called a "per se" DUI. And in most cases, the prosecution will have chemical test results showing how much alcohol or drugs were in the driver's system at the time of the arrest. If the test results show an unlawful BAC, the prosecution doesn't need to prove actual driver impairment or rely on FST results for any other reasons. Generally, BAC test results are enough to prove a per se DUI charge.
Impairment DUI charges. It's also illegal in every state to operate a vehicle while actually impaired by drugs or alcohol. In some cases, BAC test results aren't available for one reason or another, so proving impairment is necessary to get a conviction. For prosecutors trying to prove an impairment DUI charge, FST performance can be useful. In these situations, the prosecutor would call the arresting officer as a witness at trial to testify about his or her observations during the FSTs that might indicate impairment.
FST performance and issues of probable cause. In some cases, defendants argue a DUI arrest wasn't lawful because there wasn't enough evidence of impairment to supply the arresting officer with probable cause. In these cases, the prosecution might use officer testimony regarding FST performance to show the arrest was supported by probable cause.
Admissibility of FST performance. Generally, an officer's FST observations are admissible evidence in court. However, in some cases, judges might require expert testimony related to the significance of FST results before admitting officer FST observations. In other words, an expert would need to explain how and why the FST impairment clues provide evidence of driver intoxication. For example, courts in many states have held that the correlation between abnormal eye movement and impairment (an impairment clue for the horizontal gaze nystagmus test) isn't common knowledge. So, for these observations to be admissible in court, a qualified expert must testify as to this correlation.